Radio telephony, generally speaking, involves the use of portable radios for telephone communication by a user through a radio telecommunication network. The network connects a large number of network subscribers with each other and, usually, with subscribers of other networks as well. Connections between calling and called parties are made using a network infrastructure that includes information channels and switching devices to route calls to the appropriate destination. Connected subscribers may engage in voice conversations or exchange text messages, email, or other forms of data.
In a radio telecommunication network, the only actual radio-frequency connection may be that between the subscribers' radios and the network infrastructure (although this is not necessarily true). Infrastructure nodes are often fixed in location and interconnected using wires, cables, or optical fibers so that they can transfer large amounts of information. The radio connection to the subscribers is important, however, because it gives them mobility. In ideal circumstances, a subscriber can make a call and maintain the connection even when traveling over dozens, or even hundreds of miles.
In order for such communications to occur, radio telecommunication networks typically employ a large number of fixed base stations spread over a wide geographic area, sometimes referred to as the network coverage area. Each base station employs one or more antennas for communicating with nearby mobile stations, and of course is also connected to the remainder of the infrastructure as well.
FIG. 1 is a simplified block diagram illustrating selected components of a typical radio telecommunication network 100. Base stations 105-110 are each shown to be connected with an antenna 111-116. Each antenna is intended to handle communications within a selected area, sometimes referred to as a cell. (For this reason the portable subscriber radios used in such a network are often called “cellular” or simply “cell” phones.) For example, in FIG. 1 cell phones 11, 12, and 13 are shown to be in communication with antennas 111, 112, and 113, via radio channels 1, 2, and 3, respectively.
The broken lines in FIG. 1 represent cell boundaries. These boundaries do not represent the precise range of their associated antennae, of course, and are not always regular in shape or consistent in size. And although only six cells are delineated, there are typically many more in the network coverage area. Cell phones may and often do move from cell to cell, and their network communications are generally transferred from one network antenna to another though a process called handover.
Base station controllers (BSCs) 120 and 125 are in communication with, and generally control the operations of base stations 105-107 and 108-110, respectively. The base station controllers are in turn connected with a mobile switching center (MSC) 130, which handles call routing and provides a connection to other network MSCs (not shown) and gateway MSCs such as G-MSC 135, which may provide a connection to another network. A visitor location register, here VLR 140, maintains information relating to cell phones in the area services by the associated MSC. (A home location register (HLR) (not shown), may be provided to track the location and other information related to all network subscribers.)
Note that while cellular telephones have traditionally been used for voice communication, advances in technology have permitted the introduction and growing use of such instruments for other applications including sending of text messages, instant messages, data transfer, and Web surfing. Some have even incorporated functions previously performed by personal digital assistants (PDAs), such as appointment calendaring. For this reason the wide variety of such devices capable of communicating through a radio telecommunication will be referred to simply as “mobile stations”.
In radio telecommunication networks, the cellular architecture provides a number of advantages. For one, in many networks channelization for individual subscriber or control communications is implemented by assigning different frequencies to each channel. By controlling the range of these communications, assigned frequencies may be reused in non-adjacent cells without creating interference between users. In addition, the close proximity of base stations with which to communicate means that mobile stations can communicate with lower transmission power than if they had to reach distant antennas. Conservation of power is, of course, an important objective of battery-operated devices.
Frequency channelization in the mobile context is frequently referred to as frequency division multiple access (FDMA). Each mobile station is assigned one or more frequencies within the overall operational bandwidth of communicating with the base station. In some systems, each communication frequency is also divided into time slots, a scheme referred to as time division multiple access (TDMA). In TDMA, each mobile station is assigned one or more of these slots and transmits a portion of its information in turn. Naturally, the slots are of sufficient duration and frequency so that each user perceives their own conversation as continuous.
Another type of multiple access scheme is called code-division multiple access (CDMA). CDMA operates somewhat differently; rather than divide the available transmission bandwidth into individual channels, many individual transmissions are spread over a frequency band using a spreading code. Transmissions intended for a particular receiver (i.e. mobile station) are spread with spreading code assigned to the mobile station, which decodes only that information intended for it and ignores the differently-coded transmissions intended for others. The number of mobile stations that can operate in a given area is therefore limited by the number of unique encoding sequences available, rather than the number of frequency bands. The operation of a CDMA network is normally performed in accordance with a protocol referred to as IS-95 (interim standard-95) or, increasingly, according to its third generation (3G) successors, such as those sometimes referred to as CDMA2000, 1xEV-DO, and 1xEV-DV, the latter of which provides for the transport of both data and voice information.
In a more recently developed scheme, the use of CDMA techniques is combined with orthogonal frequency division multiplexing (OFDM). OFDM is a modulation method in which multiple user symbols are transmitted in parallel using a large number of different sub-carriers. These sub-carriers, sometimes called frequency bins, are used to spread transmitted information with respect to frequency rather than time (as with conventional CDMA). This multiple access scheme is sometimes referred to as multi-carrier CDMA (or MC-CDMA).
FIG. 2 is a simplified block diagram illustrating selected components of a typical MC-CDMA telecommunication system 200. As depicted in FIG. 2, system 200 has a transmit side 205 and a receive side 210. On the transmit side information, which may be either voice or data for transmission, is first encoded in encoder 215. The encoded information is passed to modulator 220 for modulation according to one of several modulation schemes such as QPSK or 16 QAM. The modulated symbols are then provided to an MC-CDMA transmitter 225 for transmission over an air interface radio channel 230.
The transmitted information is received on the receive side by an MC-CDMA receiver 235, which processes the information and presents it to detector 240 for symbol detection. Simply stated, detector 240 attempts to faithfully reconstruct the transmitted symbol stream by removing from the received signal the effects of any distortion or noise added in transmission. In part, these undesirable but unavoidable effects are removed, or at least mitigated, by analyzing the quality of certain received symbols called pilot symbols. These pilot symbols are not part of the transmitted user information, but are inserted into it. Their transmitted value is known to the receiver, which can estimate channel effects from the condition in which they are received. The detected symbol stream is then presented to decoder 245 for decoding. The decoded information is stored or provided to a user interface such as a speaker (not shown) so that it may be perceived by the user.
Another form of interference that may distort transmitted radio communications is referred to as inter-symbol interference (ISI). ISI arises largely from the multipath effect, a phenomenon that occurs when a propagating radio signal fans out and encounters, for example, different reflecting surfaces and propagation media creating a number of ‘copies’ of the same signal that may each arrive at the receiver at slightly different times. Transmit filters, such as pulse-shaping filters, that are used to limit the frequency content of the transmitted signal can also introduce ISI.
Channel equalizers in the receiver are often used to counter ISI induced by the multipath-effect. For ISI caused by the transmit filter, a matched filter may be implemented in the receiver to create an ISI-free composite filter. This approach is not always taken, however. For example, the pulse-shaping filters specified in CDMA standards such as IS-95 and 1xEv-DV are not ISI-free. As a result, the pulse-shaping filters in these systems introduce unabated ISI that degrades the ability of the receive side to accurately estimate the channel and detect the transmitted symbols.
Needed, therefore, is a way to reduce or eliminate ISI effects in MC-CDMA systems that use pulse-shaping filters in order to improve system performance. The present invention provides just such a solution.